Field of the Invention
The present invention relates to a vibration wave motor, and particularly, relates to a vibration wave motor for linear drive, in which an elastic body is formed into a plate shape.
Description of the Related Art
Heretofore, vibration wave motors, which have features such as small lightweight, high-speed drive and silent drive, have been employed for a lens barrel of an imaging device. Among them, with regard to a vibration wave motor for linear drive, a vibration wave motor as follows is disclosed in Japanese Patent Application Laid-Open No. 2004-304887.
The vibration wave motor disclosed in Japanese Patent Application Laid-Open No. 2004-304887 is composed of: a vibration plate having a rectangular flat plate portion and protruding portions provided on the flat plate portion; a piezoelectric element that is fixed to the vibration plate and performs high-frequency vibration; and a friction member brought into contact with the protruding portions. In the vibration plate, by the high-frequency vibration of the piezoelectric element, there are excited a primary natural vibration mode of a bending vibration in a short-side direction and a secondary natural vibration mode of a bending vibration in a long-side direction, in which resonance frequencies substantially coincide with each other.
Between the two natural vibration modes, in the primary natural vibration mode of the bending vibration in the short-side direction, tip ends of the protruding portions vibrate in a vertical direction with respect to the flat plate portion, and in the secondary natural vibration mode of the bending vibration in the long-side direction, the tip ends of the protruding portions vibrate in a horizontal direction with respect to the flat plate portion. These two natural vibration modes are combined with each other, whereby elliptic motion is generated in the protruding portions of the vibration plate. The protruding portions of the vibration plate are pressurized to the friction member and are brought into contact therewith, and the vibration plate and the friction member move relatively to each other in the long-side direction by friction generated by the above-mentioned elliptic motion.
In general, in a case of generating the elliptic motion by combining the two natural vibration modes as in the vibration wave motor in Japanese Patent Application Laid-Open. No. 2004-304887, then the vibrations become unstable at a frequency approximate to the resonance frequencies, and accordingly, a higher frequency range than those of the two natural vibration modes has been used for the drive. In a case where there is a difference between maximum amplitudes of the two natural vibration modes, and where the resonance frequencies the two natural vibration modes are allowed to substantially coincide with each other, then an amplitude of the primary natural vibration mode in the short-side direction becomes larger than an amplitude of the secondary natural vibration mode in the long-side direction in a frequency range at a time of the drive. Therefore, the elliptic motion generated by the two natural vibration modes has become vertically oriented elliptic motion, and a speed thereof has decreased.
As described above, there has been a problem that the speed decreases in the case where the difference is present between the maximum amplitudes of the two natural vibration modes for use in the drive of the vibration wave motor, and where the resonance frequencies of the two natural vibration modes are allowed to substantially coincide with each other.